1. Field of the Invention
The invention relates to a method for controlling the speed of horizontally rotatable supply turntables for tape-shaped material which is taken from the inside convolution of a tape reel, by means of contactless detection of the position of the tape removed, and to a device for carrying out the method.
2. Description of the Prior Art
The speed of horizontally rotatable supply turntables for tape-shape material must be continuously corrected during the unwinding of the tape strip from the inside convolution of the reel resting thereupon if the speed of the tape strip removed must be kept constant during the unwinding process. In addition, fine regulation of the speed is required if the point of separation of the tape strip removed at the inside convolution fluctuates or twists in the direction of the circumference of the rotatable supply turntable. The motor drive for the supply turntable must be controlled in accordance with the required speed control.
The invention especially relates to a device for carrying out this method during the removal of a film tape, to be shown in a film projector, at the inside convolution of a film tape reel placed on a film turntable disc, in which arrangement the film strip removed can be supplied in the form of a tape loop to an insertion unit arranged centrally fixed in location concentrically with the turntable disc.
From DE 85 12 510 U1, a device for guiding out a film tape is known in which a horizontally deflectable load arm has at its free end two tape guides between which the tape removed at the inside convolution is fed in. The load arm swivels from its radially aligned starting position in accordance with the respective point of separation of the film tape from the reel and during this action operates, depending on the shape of the tape loop and on the direction from which it is fed in leading or lagging, one of two microswitches arranged on the insertion unit at a distance from its center axis. The microswitch operated in each case controls the electric motor driving the film turntable disc, in such a manner that the speed of the film turntable disc is increased or reduced until the tape loop is fed in radially once more.
The disadvantageous factor in this known type of construction is that the film turntable device must be constructed absolutely horizontally and that the microswitches must be accurately adjusted and frequently readjusted. Since, furthermore, the film strip taken from the reel must mechanically move a lever, the latter must be supported for easy movement.
From German patent specification 12 37 893, a device for simultaneously winding up and unwinding a film tape to be shown in a film projector and having a speed control is known, in which, on a stationary central support disc, two light barriers with an angular offset of about 90.degree. and thus arranged with mutual spacing are provided in its circumferential area. The film strip passes either one or the other light barrier, depending on the point of separation of the tape loop from the inside convolution, and there triggers a corresponding switch contact for correcting the servo motor for the film turntable disc.
Such an arrangement of light barriers disadvantageously leads to misinterpretations in the event of a disturbed feed-in of the film strip, for example if more than one turn becomes detached from the reel due to splices in the film. Novel polyester film also leads to disturbances.
From U.S. Pat. No. 3,823,890 it is known to control the speed of a turntable disc for reels resting thereupon in dependence on the tension of the tape which is determined via the contactlessly sensed swivel position of a lever guiding the tape.